Reflective film interface to restore transverse magnetic wave contrast in lithographic processing

ABSTRACT

A method and system for exposing a resist layer with regions of photosensitivity to an image in a lithographic process using a high numerical aperture imaging tool. There is employed a substrate having thereover a layer reflective to the imaging tool radiation and a resist layer having a region of photosensitivity over the reflective layer, with the resist layer having a thickness. The imaging tool is adapted to project radiation containing an aerial image onto the resist layer, with a portion of the radiation containing the aerial image passing through the resist layer and reflecting back to the resist layer. The reflected radiation forms an interference pattern in the resist layer of the projected aerial image through the resist layer thickness. The thickness and location of the resist layer region of photosensitivity with respect to the reflective layer are selected to include from within the interference pattern higher contrast portions of the interference pattern in the direction of the resist thickness, and to exclude lower contrast portions of the interference pattern in the resist thickness direction from said resist layer region of photosensitivity, to improve contrast of the aerial image in said resist layer region of photosensitivity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the manufacture of integratedcircuits and, in particular, to a method and system for restoringtransverse magnetic wave contrast in features made by a lithographicprocess.

2. Description of Related Art

Manufacturers of microelectronic circuits are continually seeking toproduce features having smaller dimensions. The lithographic productionof such features typically uses a step-and-scan imaging tool 20, asshown in FIG. 1, to project a pattern onto a photosensitive resist layeron a substrate or wafer. The projection optical system of the imagingtool includes a lamp, laser, or other optical source 22 that projectsradiation 24 used to illuminate a photomask or reticle 28 through acondenser lens system 26. The photomask or reticle 28 contains thepattern to be projected and reproduced on the wafer substrate, and isgenerally oriented substantially perpendicular to an optical axis 24 ofthe projection optical system. Some of the light radiation 46 thatpasses through the photomask 28 is collected by the projection optics 34and the aerial image 36 of the pattern produced by passage of radiation46 through the mask is directed onto the wafer 42, so as to create thepattern or image 40 on the wafer.

In a step-and-scan system, the photomask 28 and the wafer 42 are mountedon mask stage 33 and substrate stage 38, respectively, that moverelative to the fixed optical system. The optical system contains anaperture or slit 32 through which light is allowed to pass to thereticle. The entire mask pattern within the desired transfer region of areticle is completely exposed by scanning along the one-dimensional scandirection 30 and across the complete one-dimensional width of thetransfer region to produce a complete pattern 40 on the wafer resist,for example a complete chip pattern. The scanning process issubsequently repeated to produce the desired number of patterns on thewafer 42.

In order to produce features having smaller dimensions in themanufacture of microelectronic circuits, three factors, aphenomenological process resolution factor (k1), the light wavelength(λ) and the numerical aperture value (NA) are involved in thelithographic processing that may be used to create the minimum linewidth (W_(min)) according to a standard generalization of Rayleigh'sequation:W _(min) =k ₁ λ/NASometimes a slightly different value of k1 is used that relates λ and NAto the half-pitch of a periodic system of lines and spaces.

To enable use of finer features in integrated circuits many advances ofbeen made in lithographic technology that allow smaller values of k1. Inthe early days of integrated circuit manufacture only k1 values above 1were practical, but now k1 values of 0.4 are being employed, and furtherreductions are sought. A difficulty here is that image contrast isdegraded at such low k1 values, making it difficult to achieve sizeuniformity in the printed circuit features as distributed over the chip,such size uniformity usually being required for acceptable circuitperformance.

Looking at the NA (numerical aperture) factor, recent advances haveenabled exposure tool manufacturers to ship tools with NA values inexcess of 0.70, 0.80 and higher, and tools with NA values of 0.93 arenow available. NA values higher than 1.0 are expected in the future,based on immersion imaging. Because modern exposure tools have such highNA values, images must be formed using waves with high angles ofpropagation within the resist, i.e., large propagation angles withrespect to a direction normal to the surface of the resist layer. Suchhigh angles of propagation may be considered to be those in excess ofabout 30°, since the orientations of the associated electric fieldvectors will then show significant variation, in that, for example, theelectric field of a ray that has propagated from one side of the lensaperture into the resist can differ in its orientation by as much as 60°from the electric field of a ray that has entered the resist afterexiting the opposite side of the lens aperture. It should be noted thatresist refractive indices are usually greater than about 1.6, so that apropagation angle of 30° within a resist layer corresponds to anincidence angle for the incoming wave above the resist that is greaterthan 50° (NA=0.8) if the incident medium is air. Thus, incidence anglesgreater than about 50° may be considered to be high incidence angleswhen the incident medium is air, depending on the resist index. However,in an immersion system, where the incident medium might have arefractive index of 1.4, incidence angles greater than about 35° couldbe considered to be high incidence angles, since they too would producea propagation angle of 30° or more within a resist layer, depending onthe resist index.

At the high numerical apertures that produce such incidence angles, ithas been observed that there is a fundamental loss of image contrast forthe transverse magnetic (TM) polarization of the light waves. Even ifthe source radiation can be transverse electric (TE) polarized for thedominant interfering orders, i.e., from a tangentially polarized source,other orders will generally be present that interfere with fields thatare partly TM polarized.

In FIG. 2, there is shown the passage of high incident angle light wavesthrough resist layer 50 on either side of a line 47 normal to thesurface of the resist layer. As used herein, the term “light” refers tothe radiation used in the lithographic imaging system, regardless ofwavelength. Incident waves 46 a and 46′a enter the resist layer atopposite angles θ, and are refracted as waves 46 c, 46′c, respectively.Incident light rays 46 a, 46′a carry a high resolution image, and sohave a high incidence angle θ with respect to the direction normal tothe surface of resist layer 50 disposed on the wafer. The directionnormal to the substrate will be referred to as the z direction. Therefractive index of a typical resist layer is generally greater thanabout 1.6 and often greater than about 1.7, in the range of about 1.6 to1.8, while the incident medium has a refractive index that is usually1.0 and in general less than the resist index, so that the angle of therefracted light rays within the resist is generally reduced, i.e.,becomes more vertically oriented. The TM polarized electric fieldvectors from these incident light rays are shown by arrows 46 b and46′b. These electric field vectors are perpendicular to the propagationdirections of their respective waves, as required by Maxwell'sequations. These interfering electric field vectors are roughlyanti-parallel, corresponding to a dark portion of the image.

During half the optical cycle, electric fields 46 d and 46′d on therefracted waves are oriented at an angle with a partially downwarddirection within the resist layer. Because the vectors share a common zcomponent (downward during the part of the optical cycle shown, upwardduring the other half of the optical cycle, and in general with the samesign), the two vector fields do not completely cancel when superposed,the z components being parallel rather than anti-parallel, and thus thedark portion of the image is not fully dark, and so contrast is reduced,which in turn reduces the controllability of the imaging process, andthus increases the minimum practical k1. The essential problem here isthat the lack of complete destructive interference at high NA valuesresults in reduced contrast from the TM polarization component. This isconventionally regarded as a problem that is inherent to high NAimaging, due to the geometry of the propagation angles involved. Theassociated contrast loss adds to the already significant contrast lossthat arises even when the NA value is not large if one forms images atlow k1, as is desirable to maximize resolution.

In a similar way, full constructive interference cannot take place inbright regions of a high NA image having nonzero TM component, becausethe TM polarized electric fields are not fully parallel when propagationangles are steep. This reduces image brightness in TM polarization.

Neither contrast nor brightness is degraded in TE polarization (notshown), in which the electric field is polarized perpendicular to theplane of incidence onto the wafer. However, the printing of circuitpatterns generally causes the waves that are incident on almost everysmall region of the wafer to have planes of incidence that are orientedin many possible azimuths. This is because most circuit patterns arepopulated fairly densely with fine circuit features in more than oneorientation. This makes it impossible to achieve pure TE polarizationfor all features using a single exposure, and multiple exposuressignificantly increase cost. Moreover, even if one could theoreticallyemploy waves that contained only a single polarization component, suchas TE, the polarization can be distorted in a number of ways, forexample by residual birefringence in the mask substrate and lens,diffractive effects in the mask, and thin-film effects in the lens andwafer process films. These polarization distortions can in turn causevariations in the dose that is delivered to different features in theimage, since TE and TM peak brightnesses differ. In general, whendiverse fine features are present with different orientations, theproportion of TM polarization to TE polarization will vary, and since TEpolarization exhibits complete constructive interference, the totaldelivered dose will vary as the proportion of TM polarization to TEpolarization varies. Low k1 imaging is particularly sensitive to dosevariations. The sensitivity to polarization distortion is reduced ifunpolarized light can be employed, even though unpolarized light can beunderstood as an incoherent superposition of TM and TE polarized light.However, for successful printing using unpolarized light, it isdesirable that both polarizations provide high contrast, and that theresist system not exhibit differential polarization sensitivity thatwould in effect remove the unpolarized condition.

SUMMARY OF THE INVENTION

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a method andsystem for improving contrast of the image of a feature projected ontothe resist layer in a lithographic process when using a high numericalaperture imaging tool.

It is another object of the present invention to provide a method andsystem for restoring transverse magnetic wave contrast in lithographicprocessing.

A further object of the invention is to provide a method for carryingout high numerical aperture lithographic processing using light that issubstantially unpolarized.

Yet another object of the invention is to provide a method for carryingout high numerical aperture lithography that is insensitive to changesof polarization induced by the mask, imaging optics, or wafer processfilms.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The above and other objects, which will be apparent to those skilled inthe art, are achieved in the present invention which is directed to amethod of exposing a resist layer with regions of photosensitivity to animage in a lithographic process using a high numerical aperture imagingtool comprising providing an imaging tool having a high numericalaperture for projecting radiation forming an aerial image, and providinga substrate having a layer reflective to the imaging tool radiation anda resist layer with a region of photosensitivity over the reflectivelayer, the resist layer having a thickness. The method then includesprojecting radiation containing an aerial image from the imaging toolonto the resist layer, a portion of the radiation containing the aerialimage passing through the resist layer, and reflecting a portion of theradiation containing the aerial image that passes through the resistlayer off the reflective layer back to the resist layer. The reflectedradiation forms an interference pattern in the resist layer of theprojected aerial image through the resist layer thickness. The locationof the resist layer region of photosensitivity with respect to thereflective layer is selected to include from within the interferencepattern higher contrast portions of the interference pattern in thedirection of the resist thickness, and to exclude lower contrastportions of the interference pattern in the resist thickness directionfrom said resist layer region of photosensitivity, to improve contrastof the aerial image in said resist layer region of photosensitivity.

The radiation includes transverse magnetic waves and the reflectivelayer is reflective to the transverse magnetic waves, and theinterference pattern in the resist layer region of photosensitivityincludes higher contrast portions of the interference pattern created bythe transverse magnetic waves.

In another aspect, the present invention is directed to a system forexposing a resist layer with regions of photosensitivity to an image ina lithographic process using a high numerical aperture imaging toolcomprising a substrate having thereover a layer reflective to theimaging tool radiation and a resist layer having a region ofphotosensitivity over the reflective layer, with the resist layer havinga thickness. The imaging tool is adapted to project radiation containingan aerial image onto the resist layer, with a portion of the radiationcontaining the aerial image passing through the resist layer andreflecting back to the resist layer. The reflected radiation forms aninterference pattern in the resist layer of the projected aerial imagethrough the resist layer thickness. The thickness and location of theresist layer region of photosensitivity with respect to the reflectivelayer are selected to include from within the interference patternhigher contrast portions of the interference pattern in the direction ofthe resist thickness, and to exclude lower contrast portions of theinterference pattern in the resist thickness direction from said resistlayer region of photosensitivity, to improve contrast of the aerialimage in said resist layer region of photosensitivity.

The resist layer thickness with respect to the reflective layer ispreferably selected to include in the interference pattern a singlehigher contrast region of a transverse magnetic wave interferencepattern in the resist thickness direction, and to exclude lower contrastregions of the transverse magnetic wave interference pattern from theresist layer in the resist thickness direction, to improve contrast ofthe aerial image in the resist layer. The resist layer preferablyincludes an inorganic moiety selected from the group consisting ofsilicon, iron, hafnium and titanium, and combinations thereof.

A spacer layer is preferably included between the reflective layer andthe resist layer. The spacer layer preferably comprises an organiccopolymer and has a refractive index of about 1.7.

The resist layer preferably has a thickness less than about:(2*k1*λ/NA)/Sqrt[(4*k1*n/NA)ˆ2−1],

where k1 is the half-pitch k1-factor of the process, and n is therefractive index of the resist.

More preferably, the resist layer has a thickness of less than 30 nm,for example about 15 to 30 nm, and is spaced from the reflective layerby a spacer layer having a thickness of at least 60 nm.

The method is particularly useful where the imaging tool has a numericalaperture value of at least 0.95.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a perspective view of a step and scan exposure system forprojecting a masked image onto a wafer.

FIG. 2 is a cross sectional elevational view of the refraction of highincident angle TM polarized light rays into the resist layer of a wafer.

FIG. 3 is a cross sectional elevational view of the passage of highincident angle light rays through a wafer resist layer and reflectedfrom a reflective layer on the wafer substrate.

FIG. 4 is a cross sectional elevational view showing the changes incontrast in the x- and z-directions produced in a resist layer in TMpolarization as a result of a reflective layer on the wafer substrate.

FIG. 5 is a cross sectional elevational view showing the selection tomaximize TM contrast of resist photosensitivity location with respect toa reflective layer, and selection of resist thickness, in accordancewith the present invention.

FIG. 6 shows the selected resist photosensitivity location andthickness, in accordance with the present invention.

FIG. 7 shows graphical representations of the performance of theinvention in substantially restoring TM contrast when printing low-k1lines and spaces.

FIG. 8 shows graphical representations of the performance of theinvention in maintaining TM contrast when printing moderate-k1 lines andspaces.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 3-8 of the drawings in which likenumerals refer to like features of the invention.

To improve contrast in an image during exposure of the resist layer in alithographic process when using a high numerical aperture imaging tool,the present invention employs reflective interference of transversemagnetic (TM) waves that have z components which are oppositely orientedto those of the incoming image waves, and selection of a resist layerthickness and photosensitivity location that are appropriately matchedto the z interference pattern, above a layer reflective to the radiationemployed in the imaging tool.

In FIG. 3, there is shown the passage of high incident angle light wavesthrough resist layer 50. Incident waves 46 a and 46′a enter the resistlayer at opposite angles θ, and are refracted as waves 46 c, 46′c,respectively. The TM polarized electric field vectors from theseincident light rays are shown by arrows 46 b and 46′b. During half theoptical cycle, electric fields 46 d and 46′d on the refracted waves areoriented at an angle with a partially downward direction within theresist layer, i.e. a downward pointing component along the z axis.

In accordance with the present invention, FIG. 3 shows a configurationthat provides between resist layer 50 and wafer substrate 42 a layer 60which is reflective to the light utilized in the imaging system. Itshould be noted that FIG. 3 does not show a preferred rendering of thepreferred embodiment of the invention even though it includes reflectivelayer 60, since resist layer 50 is of conventional form. Typically, forradiation of wavelength of about 157 nm to 365 nm, the invention employsa reflective system of layers that may comprise an aluminum film ofabout 100 nm thickness. Each light ray 46 c, 46′c is reflected off ofreflective film 60, producing reflected waves 46 e and 46′e,respectively. These reflected rays have TM vectors 46 f and 46′f,respectively, which are oriented at an angle in a generally upwarddirection during the half of the optical cycle in which fields 46 d and46′d are oriented generally downward. At the z position shown, thevector sum of the 4 vectors 46 d, 46′d, 46 f, and 46′f is almost zero,because of the geometrical orientation of these fields and the highreflectivity of the reflective layer. In other words, destructiveinterference is almost complete at the z position shown, because the zcomponents of the reflected waves 46 e and 46′e that have been producedby the reflective layer are oppositely oriented to the z components ofthe incident image waves 46 c and 46′c, and have almost equal magnitude.More generally, as a result of the interference of TM rays 46 d, 46′dwith 46 f, 46′f, an interference pattern is formed in the z-direction,i.e., through the thickness of resist layer 50. While a nearly completedestructive interference can in principle be achieved, perfectconstructive interference is not achieved in all four TM waves shown. Inthis configuration, vector interference reduces peak dose in TMpolarization compared to TE polarization, but within certain planes doesnot reduce TM contrast. However, even though, as in the prior art,constructive interference is incomplete, the new configuration usuallycauses overall TM dose to increase within the planes of high TMcontrast, since the resist is now exposed by both incident and reflectedwaves. A dose variation in the z-direction occurs in both TE and TMpolarizations. Planes of maximum TM dose are also planes of maximum TMcontrast.

FIG. 4 shows the interference pattern created by such high incidentangle TM waves through the thickness of resist layer 50, as a result ofreflection from reflective film 60. The z-direction is through thethickness of the resist layer (shown having a thickness of 250 nm) andthe x- and y-direction is in the plane of the resist layer. In thisfigure, nominally dark line 52 is formed at x-position of approximately0, and bright space 54 is formed at x-position of approximately −80 nm.The interference patterns are also shown for additional dark lines (notidentified) at x-positions of approximately +160 nm and −160 nm, and fora bright space (not identified) at x-position of approximately +80 nm.The dark lines and bright spaces continue to repeat outside theboundaries of the figure. Along dark line 52, the lighter regions 80that are spaced apart in the z-direction are regions of less contrast(where the line would be seen to be less dark), separated byspaced-apart regions where the contrast is at a maximum (where the linewould be seen to be darker). For example dark line 52 has less contrastregions 80 at z-positions of approximately 13 nm, 88 nm, 163 nm and 238nm, and has regions of maximum contrast at heights of approximately 50nm, 125 nm, and 200 nm. For bright line 54, the high intensity regions90 with higher intensity centers 92, spaced apart in the z-direction,are seen as corresponding to the high contrast regions of dark line 52.

FIG. 5 shows the interference patterns for both TM and TE polarizedwaves produced in a resist layer 50 of conventional thickness c by areflective layer. As will now be explained, FIG. 5 can also beunderstood as including a rendition of the invention that has beenannotated with other features to help explain the functioning of theinvention. The resist layer of conventional thickness 50 has TMinterference pattern 101 with low contrast region 80 along with highintensity region 90 and region 92 of even higher intensity, and TEinterference pattern 111 with comparable high intensity regions 90′ and92′, but with uniformly high contrast. To exploit the completedestructive interference obtained with underlying reflective layer 60,in accordance with the present invention the portion of the resist thatwould have the highest TM contrast in the z-direction thickness isutilized for the resist layer to be employed on the wafer. In otherwords, a resist layer of less than conventional thickness is positionedat a desired location above reflective layer 60, and with a suitablethickness, so that it includes the interference pattern portions havinghigher contrast, and excludes at least some, preferably most, of thoseinterference pattern portions that would have a lower contrast. Ingeneral, one matches the location and thickness of the resistphotosensitivity region or regions with the high contrast regions of theinterference pattern along z.

A preferred resist layer selection is shown in FIG. 5, in which a thinimaging layer photosensitive portion 50′ having thickness a at adistance b above reflective layer 60 is selected as the resist layerthickness and location to be utilized. Photosensitive resist layer 50′is seen as including the TM interference portion 102 having the highestcontrast from TM interference pattern 101, and excludes those portionsof the interference pattern that have lesser TM contrast. While thethickness a of photosensitive resist layer 50′ is shown encompassingonly a single region of high TM contrast in the z-direction, multipleregions of high TM contrast in the z-direction may be used if the upperand lower adjacent regions of lesser contrast are excluded. To ensurethe proper location of the photosensitive resist layer 50′ with respectto reflective layer 60, a spacer layer 70 of thickness b may beemployed. The z-direction thickness of photosensitive resist layer 50′is typically thinner than normally used, to confine the resist exposureto only a single region of unreversed TM contrast within what wouldnormally be the full interference pattern in a thicker resist layer 50.

The selected TM interference pattern 102 is shifted vertically relativeto the TE interference pattern portion 112 in photosensitive resistlayer 50′, due to differential TE/TM phase shift from the reflectivelayer on the substrate in the presence of the spacer medium 70. Thephotosensitive resist layer 50′ may also be selected by its location toexclude part of the TE bright fringe (which is actually brighter thanthe TM intensity, but plotted here with an independent brightnessscale), in order to approximately match the collected TE dose with theTM dose. Since the system then provides substantially equal doses in TEand TM polarization as well as substantially equal contrasts, it may beused with unpolarized light, and is not vulnerable to the polarizationsensitivities described above. Together, the spacer layer and reflectivefilm comprise a reflective layer system. Other highly reflective layercombinations can be used, so long as the reflective layer system is thinenough to remain substantially within the folded depth of focus of theimaging system, where the TM phase shift positions the photosensitiveresist portion near a plane of high TM contrast, and where preferablythe TE/TM differential phase shift approximately balances the TE and TMdoses within the photosensitive region. Usually the last of these designobjectives is less important than the other two. The TE and TMinterference patterns of FIG. 5 are only schematic in such embodiments,but a qualitatively very similar analysis applies in general as far asthe interfering waves within the photosensitive regions are concerned.

A quantitative analysis can be carried out using the equations providedby Rosenbluth et al. in “Fast calculation of images for high numericalaperture lithography,” SPIE v.5377—Optical Microlithography XVII,(2004): p. 615. Using eq.11 or eq.19 of that reference, one can showthat with the proper spacer thickness the TM contrast ratio (CR) isgiven by${CR} = \frac{1 - {2\sin^{2}\theta^{''}\frac{1 + {r}^{2}}{\left( {1 + {r}} \right)^{2}}}}{1 - {4\sin^{2}\theta^{''}\frac{r}{\left( {1 + {r}} \right)^{2}}}}$

Here θ″ is the angle of propagation inside the resist film, and r isdefined asr≡v_(p)/u_(p)

r is essentially the reflectivity of the reflective film stack whenplaced within the wafer film stack. v_(p) and u_(p) are the amplitudesof the up-traveling and down-traveling TM waves, respectively, per unitincident wave amplitude. v_(p) and u_(p) may be calculated usingstandard thin film methods, as explained in Rosenbluth et al. The valuer is particularly easy to calculate when the resist layer can beapproximated as a dielectric; in that case one can simply ignore allfilms above the resist layer, and treat the incident medium as asemi-infinite film of index equal to the resist index. CR in theseequations is defined as:(I_(max)−I_(min))/(I_(max)+I_(min)),

where I_(max) is the intensity of a bright space, and I_(min) theintensity of a dark line, under standard lithographic conditions forproducing two-beam line/space images. When the spacer thickness is notoptimal the phase of r changes, and CR may be calculated from:$\frac{I_{\min}}{I_{\max}} = {\frac{1 - {CR}}{1 + {CR}} = {\tan^{2}\theta^{''}\frac{1 - {2\frac{r}{1 + {r}^{2}}{\cos\phi}}}{1 + {2\frac{r}{1 + {r}^{2}}{\cos\phi}}}}}$

where θ is the change in phase of r.

In conventional resist systems r is fairly small, and thus CR≈1−2*sin²θ″ conventionally; clearly CR can be degraded quite substantiallywhen the propagation angle inside the resist is large. On the otherhand, if |r|≈1, CR≈1 when φ=0. Moreover, the difference between the 2|r|factor appearing in the denominator of the above CR equation and the1+|r|² expression in the numerator will decrease quadratically as |r|approaches 1, which means that very substantial contrast improvement ispossible even when |r| is only moderately large.

The photosensitive resist layer used in accordance with the presentinvention preferably has a thickness less than about(2*k1*λ/NA)/Sqrt[(4*k1*n/NA)ˆ2−1],

where k1 is the half-pitch k1-factor of the process, and n is therefractive index of the resist. More preferably, the photosensitiveresist layer thickness is less than about half this amount, in the rangeof about 15 to 30 nm.

For clarity the components of the invention in the FIG. 5 embodiment areshown in isolation in FIG. 6, with the TE and TM interference patternsremoved.

In a preferred embodiment, the photosensitive resist layer has as highan index of refraction as possible, where n is preferably greater than1.85, and has hardmask properties for etch resistance when used in athin layer in accordance with the present invention. Suitable materialsshowing sufficient etch resistance may be chosen from photoresistsystems containing inorganic moities. Typical examples of resist systemssuitable for 193 nm lithography where the inorganic moiety is siliconare described in U.S. Pat. Nos. 6,770,419, 6,653,048 and 6,444,408.Additional examples of resist systems where the inorganic moiety isferrocene (iron), hafnium based, or titanium based, are described inU.S. Pat. No. 6,171,757. The disclosures of these patents are herebyincorporated by reference. Although the inorganic moieties used in thisinvention may exhibit high absorbance at 193 nm lithography acombination between silicon and inorganic moities such as titanium,hafnium or ferrocene maybe used to achieve the appropriate index as wellas transparency suitable for the thin photosensitive layers employed inthis invention.

The preferred spacer used in the present invention is an organiccopolymer, such as an acrylate or polystyrene, with a refractive indexof about 1.7 when the resist film has a high index, such as above 1.85.For very high index resist, and spacer indices of about 1.7, the optimumthickness of the spacer can be about 64 nm, and can preferably be chosenin the range of about 64±5 nm.

In general, the appropriate spacer thickness for a given set of filmindices can be determined either by simulation, using for example one ofthe commercially available lithography simulation programs, orexperimentally, by spinning a spacer with a graded thickness onto a testwafer, and then determining the optimal thickness by comparing theperformance in different regions of the test wafer, in particular byfinding the region and associated spacer thickness that minimizes thedifference between TE and TM doses.

A resist layer may be photosensitive in its top portion, and it may beformed or treated in such a way that its remaining portion can be etchedthrough after the exposed areas in the photosensitive portion have beendeveloped away, supposing first that the resist is a positive-toneresist. This is also possible for negative-tone resists, except thatwith negative-tone resists it is the unexposed portions of thephotosensitive portion that are developed away. Alternatively, adifferent underlayer film may be etched through after developing animage in an upper photosensitive resist layer. The developed region andthe etched region can then serve as a stencil to transfer the patterninto a constituent film of the integrated circuit that is undergoingfabrication. In this context, the photosensitive region includes onlythose portions of the resist that both respond to the exposing image,and that respond to the post-exposure developer, since it is only thedeveloped regions whose pattern is transferred into the integratedcircuit. In many cases the resist layer is heated after being exposedbut before being developed. When the thickness of the photosensitiveregion is in the range of 15 nm to 30 nm, this heating step smoothes theexposing profile across this thickness, rendering it reasonably uniformover the z range.

Resist refractive indices of about 1.7 can be achieved when theinorganic moiety is silicon, and this chemistry is very well developed.A suitable spacer index for such a resist is n=1.6, and the thickness ofthe spacer can be about 77±5 nm. FIG. 7 shows graphical representationsof a test of this configuration of the invention made by simulating theexposure of a wafer to a line/space pattern of 90 nm pitch produced byan alternating phase shifting mask using a conventional light sourcewith poles of radius 0.1 positioned such that the ±15 orders arecentered at a pupil coordinate of 0.89 (expressed as a fraction of thepupil radius), a wavelength of 193 nm, an NA of 1.2, and a couplingindex of refraction of 1.43. This corresponds to a k1 factor of 0.28 forthe half-pitch. The wafer substrate has a reflective layer of aluminumof 100 nm thickness, a spacer layer of 77 nm thickness and refractiveindex of 1.6, and a resist layer of 20 nm thickness and refractive indexof 1.7. The plotted exposure profile is the average over the 20 nm zrange. When compared to a conventional resist layer of 160 nm thicknessand refractive index of 1.7, without the reflective layer and spacer,the present invention is seen to produce much higher TM contrast thanthe conventional resist and wafer combination, and to substantiallymatch TE and TM doses. Focus for the conventional system has been setaccording to a criterion of maximizing TM contrast, but nonetheless theTM contrast achieved by the conventional system is very poor. Theconventional system could instead be adjusted for a better TE depth offocus, but in that case the TM contrast would be even worse.

At the relatively steep propagation angles entailed by k1=0.28 imaging,the invention achieves a TM phase shift from the reflective layer systemthat provides high TM contrast in the resist layer, and a differentialTE/TM phase shift that substantially balances the TE and TM doses. As isstandard practice in the lithography art, the source parameters areoptimized around the most aggressive k1 features present, in this casethe k1=0.28 features. This optimization improves depth of focus, arequirement that the invention eases in one respect, due to the thinresist layer used, but makes more stringent in another respect, sincethe reflected waves must remain reasonably well focused. FIG. 7 showsthat, overall, the invention provides a depth of focus that isacceptable for such an aggressive k1 factor, and in particular a betterdepth of focus than the conventional system under conditions wherenon-negligible TM contrast is required.

All of these imaging requirements tend to be more easily achieved at thesmaller propagation angles that are associated with larger k1 factors.However, one must also verify that process parameters optimized for themost aggressive k1 features will perform adequately for any featureswith larger k1 factor that may be present, even though these parametersare not optimized for the larger k1 features. FIG. 8 illustrates thatthe invention achieves this, using an example where the invention isapplied to a so-called “DFM design”, in which moderate-pitch featuresare present at twice the pitch of the most aggressive k1 features.

Thus, the present invention provides improved contrast of the image of afeature projected onto the resist layer in a lithographic process whenusing a high numerical aperture imaging tool. The present inventionrestores transverse magnetic wave contrast in lithographic processing byproviding a film stack that reflects additional interfering waves intothe image in order to enable full destructive interference, and thatachieves better uniformity with respect to other polarizationdependencies such as TE versus TM dose, and permits the use ofunpolarized light.

If stray light increases from the reflective layer system, the effectmay be ameliorated by adding an absorbing topcoat, because the imagewill only be attenuated by the transmission of this topcoat, whilereflected stray light will be reduced by the square of the topcoattransmission. Alternatively, the effect of stray light may be correctedusing other known flare correction methods that have recently beendeveloped, for example those described in US20050091634A1,US20050091631A1, and US20050091014A1.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1. A method of exposing a resist layer with regions of photosensitivityto an image in a lithographic process using a high numerical apertureimaging tool comprising: providing an imaging tool having a highnumerical aperture for projecting radiation forming an aerial image;providing a substrate having a layer reflective to the imaging toolradiation and a resist layer with a region of photosensitivity over thereflective layer, the resist layer having a thickness; projectingradiation containing an aerial image from the imaging tool onto theresist layer, a portion of the radiation containing the aerial imagepassing through the resist layer; and reflecting a portion of theradiation containing the aerial image that passes through the resistlayer off the reflective layer back to the resist layer, the reflectedradiation forming an interference pattern in the resist layer of theprojected aerial image through the resist layer thickness, wherein thelocation of the resist layer region of photosensitivity with respect tothe reflective layer is selected to include from within the interferencepattern higher contrast portions of the interference pattern in thedirection of the resist thickness, and to exclude lower contrastportions of the interference pattern in the resist thickness directionfrom said resist layer region of photosensitivity, to improve contrastof the aerial image in said resist layer region of photosensitivity. 2.The method of claim 1 wherein the radiation includes transverse magneticwaves and the reflective layer is reflective to the transverse magneticwaves, and wherein the interference pattern in the resist layer regionof photosensitivity includes higher contrast portions of theinterference pattern created by the transverse magnetic waves.
 3. Themethod of claim 1 wherein the resist layer thickness with respect to thereflective layer is selected to include in the interference pattern asingle higher contrast region of a transverse magnetic wave interferencepattern in the resist thickness direction, and to exclude lower contrastregions of the transverse magnetic wave interference pattern from theresist layer in the resist thickness direction, to improve contrast ofthe aerial image in the resist layer.
 4. The method of claim 3 furtherincluding a spacer layer between the reflective layer and the resistlayer.
 5. The method of claim 1 wherein the imaging tool has a numericalaperture value of at least 0.95.
 6. The method of claim 3 wherein theresist layer has a thickness less than about:(2*k1*λ/NA)/Sqrt[(4*k1*n/NA)ˆ2−1], where k1 is the half-pitch k1-factorof the process, and n is the refractive index of the resist.
 7. Themethod of claim 3 wherein the resist layer has a thickness of less than30 nm, and is spaced from the reflective layer by a spacer layer havinga thickness of at least 60 nm.
 8. The method of claim 7 wherein theresist layer has a thickness of about 15 to 30 nm.
 9. The method ofclaim 1 wherein the resist layer includes an inorganic moiety selectedfrom the group consisting of silicon, iron, hafnium and titanium, andcombinations thereof.
 10. The method of claim 4 wherein the spacer layercomprises an organic copolymer.
 11. A system for exposing a resist layerwith regions of photosensitivity to an image in a lithographic processusing a high numerical aperture imaging tool comprising: a substratehaving thereover a layer reflective to the imaging tool radiation; aresist layer having a region of photosensitivity over the reflectivelayer, the resist layer having a thickness; wherein the imaging tool isadapted to project radiation containing an aerial image onto the resistlayer, with a portion of the radiation containing the aerial imagepassing through the resist layer and reflecting back to the resistlayer, the reflected radiation forming an interference pattern in theresist layer of the projected aerial image through the resist layerthickness, and wherein the thickness and location of the resist layerregion of photosensitivity with respect to the reflective layer areselected to include from within the interference pattern higher contrastportions of the interference pattern in the direction of the resistthickness, and to exclude lower contrast portions of the interferencepattern in the resist thickness direction from said resist layer regionof photosensitivity, to improve contrast of the aerial image in saidresist layer region of photosensitivity.
 12. The system of claim 11wherein the resist layer thickness with respect to the reflective layeris selected to include from within the interference pattern a singlehigher contrast region of a transverse magnetic wave interferencepattern in the resist thickness direction, and to exclude lower contrastregions of the transverse magnetic wave interference pattern from theresist layer in the resist thickness direction, to improve contrast ofthe aerial image in the resist layer.
 13. The system of claim 12 furtherincluding a spacer layer between the reflective layer and the resistlayer.
 14. The system of claim 11 wherein the imaging tool has anumerical aperture value of at least 0.95.
 15. The system of claim 12wherein the resist layer has a thickness less than about:(2*k1*λ/NA)/Sqrt[(4*k1*n/NA)ˆ2−1], where k1 is the half-pitch k1-factorof the process, and n is the refractive index of the resist.
 16. Thesystem of claim 12 wherein the resist layer has a thickness of less than30 nm, and is spaced from the reflective layer by a spacer layer havinga thickness of at least 60 nm.
 17. The system of claim 16 wherein theresist layer has a thickness of about 15 to 30 nm.
 18. The system ofclaim 11 wherein the resist layer includes an inorganic moiety selectedfrom the group consisting of silicon, iron, hafnium and titanium, andcombinations thereof.
 19. The system of claim 13 wherein the spacerlayer comprises an organic copolymer.
 20. The system of claim 13 whereinthe spacer layer has a refractive index of about 1.7.